It is useful, for minimizing interference and distortion, to vary the carrier frequency from one burst to another, e.g. by adjusting a magnetron serving as the generator of the periodic carrier-frequency bursts. Heretofore, these adjustments were usually made in a random manner, with suitable compensating changes carried out in the associated receiving channel. These compensating adjustments, generally in the operating frequency of a local oscillator, thus had to be made rapidly and with great precision; this could be done only with rather complex circuitry.